Because of its inexpensiveness, copying materials utilizing the photosensitivity of diazo compounds have been widely used. These diazo type copying materials can be roughly divided into the following three known types.
The first type is known as wet development type. In this type, a light-sensitive layer comprising a diazo compound and a coupling component as main components is provided on a support. This copying material is superimposed on an original, exposed to light, and then developed with an alkaline solution.
The second type is known as dry development type. This type of copying material is developed with ammonia gas.
The third type is known as heat-developable type. Examples of the heat-developable type copying material include a type of copying material comprising an ammonia gas generator such as urea capable of generating ammonia gas upon heating in a light-sensitive layer, a type of copying material comprising a light-sensitive layer containing an alkali salt of a compound such as trichloroacetic acid which loses acidic properties upon heating, and a type of copying material which utilizes the activation of a diazo compound and a coupling component by heat-melting with a higher fatty acid amide as a coloring aid.
The wet development type copying material is disadvantageous in maintenance and operation, because the use of a developer has much trouble of the replenishment and disposal of the developer and an apparatus therefor is large-sized. This type of copying material is also disadvantageous in that the material which has been just after copy is still wet and thus is not writable, and images thus copied can not withstand prolonged storage.
Further, the dry type copying material also has a trouble of the replenishment of a developer. This type of a copying material is also disadvantageous in that a gas suction apparatus for preventing the resulting ammonia gas from leaking out is needed to cause a large-sized copying apparatus. This type of a copying material is further disadvantageous in that the material which has been just after copy has a strong odor of ammonia.
On the other hand, the heat-developable type copying material is advantageous in maintenance, because it requires no developer unlike the wet development type and the dry development type. However, the heat-developable type also leaves much to be desired. That is, this type of copying material is disadvantageous in that since the development requires a high temperature of 150.degree. C. to 200.degree. C. and must be controlled within .+-.10.degree. C. from the desired value to inhibit insufficient development or tone change, the cost of the copying machine must be high to obtain an excellent image quality.
In order to withstand such a high temperature development, the diazo compound to be used needs to have a high heat resistance. Such a compound often acts against the formation of a high image density. This trouble can not be avoided also in the development of black tone, which is frequently used.
Many attempts have been made to lower the development temperature to a range of 90.degree. C. to 130.degree. C. However, this approach is disadvantageously accompanied by a reduction in the shelf life of the copying material itself.
Thus, although the heat-developable type copying material is advantageous in maintenance as compared with the wet development type and the dry development type, it does not yet predominate in the diazo copying system.
In other words, in order to obtain a desired color density by heating a material comprising a layer containing a diazo compound, a coupling component and a coloring aid on a support, it is necessary that these components momentarily undergo melting, diffusion and reaction to produce developed dyes. In the case, the color development reaction proceeds gradually even during the storage at room temperature before copying, resulting in coloring and hence staining of the background, which must be white.
These problems can be almost solved by the use of a copying material comprising a heat-developable light-sensitive layer containing a diazo compound, a coupling component and a coloring aid on a support, wherein the diazo compound is incorporated in microcapsules, as described in JP-A-59-91438 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, a black-developable material which can exhibit more excellent color balance has been desired.
The inventors made extensive studies to enhance the black balance in a heat-developable recording material. As a result, it was found that extremely excellent results can be obtained by the combined use of a light-sensitive diazo compound and two or more certain kinds of couplers as couplers for the diazo compound.